United States Nuclear Regulatory Commission - Protecting People and the Environment

Information Notice No. 88-80: Unexpected Piping Movement Attributed to Thermal Stratification

                                  UNITED STATES
                          NUCLEAR REGULATORY COMMISSION
                      OFFICE OF NUCLEAR REACTOR REGULATION
                             WASHINGTON, D.C.  20555

                                 October 7, 1988


Information Notice No. 88-80:  UNEXPECTED PIPING MOVEMENT ATTRIBUTED 
                                   TO THERMAL STRATIFICATION 


Addressees: 

All holders of operating licenses or construction permits for pressurized 
water nuclear power reactors. 

Purpose: 

The purpose of this information notice is to alert addressees to unexpected 
thermal movement of the pressurizer surge line attributed to thermal strati-
fication.  It is expected that recipients will review the information for 
applicability to their facilities and consider actions, as appropriate, to 
avoid similar problems.  However, suggestions contained in this information 
notice do not constitute NRC requirements; therefore, no specific action or 
written response is required. 

Description of Circumstances: 

The licensee for the Trojan plant has observed the results of unexpected move-
ment of the pressurizer surge line at every refueling outage since 1982 when 
monitoring of the line movements began.  The monitoring program was 
implemented after removal of a thermal sleeve that was found to have cracked 
attachment welds. During the last refueling outage, the licensee found that, 
in addition to unexpected gap closures in the pipe whip restraints, one 
restraint was in actual contact with the piping although the shims and gap 
sizes had been adjusted on the basis of previous analysis.  The recent 
investigation indicated that the movement of piping was caused by thermal 
stratification in the line.  This phenomenon was not considered in the 
original piping design. 

Discussion: 

Unexpected piping movements are highly undesirable because of potential high 
piping stress that may exceed design limits on fatigue and stresses.  The 
problem can be more acute when the piping expansion is restricted, such as 
through contact with pipe whip restraints.  Plastic deformation could result, 
which could lead to functional impairment of the line.  The Trojan licensee's 
report indicated that thermal stratification might have occurred in the pres-
surizer surge line during heatup, cooldown, and steady-state operation of the 
plant.  




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                                                            IN 88-80 
                                                            October 7, 1988 
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During a typical plant heatup, water in the pressurizer is heated to about 
440xF.  A steam bubble is then formed in the pressurizer.  Although the exact 
phenomenon is not thoroughly understood, as the hot water flows (at a very low
flowrate) from the pressurizer through the surge line to the hot-leg piping, 
the hot water rides on a layer of cooler water, causing the upper part of the 
pipe to be heated to a higher temperature than the lower part.  The 
differential temperature could be as high as 300xF, based on limitations on 
plant operation.  Under this condition, analysis has shown that differential 
thermal expansion of the pipe metal causes the pipe to deflect. 

In the specific configuration of the pressurizer surge line in the Trojan 
plant, the line deflected downward, contacted two pipe whip restraints, and 
underwent plastic deformation, which resulted in the permanent deformation of 
the pipe. 

On the basis of the evaluation of the Trojan event, the staff concludes that 
the thermal stratification hypothesis for the pressurizer surge line movement 
is supportable by the observations.  The licensee for Trojan is required by 
the American Society of Mechanical Engineers Boiler and Pressure Vessel Code 
to reconcile the pipe stresses and fatigue evaluation if any significant dif-
ferences are observed between the measured data and the analytical results for
the hypothesized conditions.  The staff evaluation indicates that the thermal 
stratification phenomenon could occur in all PWR surge lines.  The staff's 
concerns include unexpected bending, and thermal striping as they affect the 
overall integrity of the surge line for its design life (e.g., the increase of
fatigue). 

No specific action or written response is required by this information notice.
However, the staff may consider further generic communications on the subject.
If you have any questions about this matter, please contact the technical 
contacts listed below or the Regional Administrator of the appropriate 
regional office. 




                                   Charles E. Rossi, Director 
                                   Division of Operational Events Assessment 
                                   Office of Nuclear Reactor Regulation 

Technical Contacts:  S. N. Hou, NRR 
                     (301) 492-0904 

                     N. P. Kadambi, NRR 
                     (301) 492-1153 

Attachment:  List of Recently Issued NRC Information Notices 
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